The development of new procedures for crop production and post-harvest processing requires models. Models based on physical backgrounds are most useful for this purpose because of their extrapolation potential. An optimal procedure is developed for alfalfa drying using a physical model. The model considers the differences in drying behaviour between stems and leaves of alfalfa, the heat and mass balances of the drying air and a model for a solar energy system. The complete model is used to calculate the dynamic optimal operation for alfalfa drying in a thin layer. The results show that most of the operation time the air flow rate is at a minimum value, so during day time the solar energy is maximally utilized while during night time the costs of additional heating are minimized.